We review some aspects of the isotope effect (IE) in superconductors. Our focus is on the influence of factors not related to the pairing mechanism. After summarizing the main results obtained for conventional superconductors, we review the effect of magnetic impurities, the proximity effect and non-adiabaticity on the value of the isotope coefficient (IC). We discuss the isotope effect of T c and of the penetration depth δ. The theory is applied to conventional and high-T c superconductors.

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... erimental results obtained for YBa 2 Cu 3 O 7 δ related materials (Zn and Pr-substituted as well as oxygen-depleted systems) and for La 2 x Sr x CuO 4 are discussed. THE ISOTOPE EFFECT: DISCOVERY, CONVENTIONAL VIEW The isotope effect of superconducting critical temperature T c is best described in terms of the isotope coefficient ( IC) α defined by the relation T c ¥ M ¢ α , where M is the ionic mass. Under the assumption that the shift ∆T c induced by isotopic substitution (M ¦ M ¡ ) is small compared to T c , one can write α § © M ∆M ∆T c T c where ∆M § M¨M ¡ is the difference between the two isotopic mass. If the superconductor is composed of different elements, one defines a partial isotope coefficient α r as in Eq. (1), but where M is replaced by M r , the mass of element r that is substituted for its isotope. In addition, one defines the total isotope coefficient by Besides T c there are other quantities that display an isotope effect. In the next sections we focus on the isotope shift of the penetration depth δ. In analogy to the IC of T c (denoted α) we define the isotope coefficient β of the penetration depth by the relation β § ©